Selective catalytic reduction (“SCR”) exhaust aftertreatment systems are an important technology for reducing NOx emissions from internal combustion engines such as diesel engines. SCR systems generally include a source of urea solution, a pump unit for pressurizing the urea solution, a metering unit for providing a controlled amount or rate of urea solution, and an injector which provides urea solution to an exhaust flowpath including an SCR catalyst. Many SCR systems also utilize pressurized gas to assist the flow of urea solution to the injector. While providing important reductions in NOx emissions, SCR systems suffer from a number of shortcomings and problems. Use of urea solutions in SCR systems may result in growth of urea crystals or deposits on various components of the system, which may disrupt their operation. When the urea comes into contact with a hot injector nozzle, the urea may undergo flash-boiling, resulting in pressure waves forcing urea backward through the blending chamber and into the pressurized air supply passage. This may result in urea crystals forming within the pressurized air supply passage, which may obstruct the air supply passage, and may corrode or otherwise degrade the walls of the air supply passage. Additionally, if an injector nozzle becomes blocked, either permanently or intermittently, there is a risk that injected urea may flow into the pressurized air supply passage which may result in urea crystal formation within the pressurized air supply passage. There is a long felt need for advancements mitigating these and other shortcomings associated with SCR systems utilizing urea solution.